The present invention is directed to a composite optically nonisotropic material, a method of manufacturing composite optically nonisotropic materials, and to devices using composite optically nonisotropic materials for optical detection. The invention is also directed to a composite optically nonisotropic material where the composite material includes a transparent polycrystalline ceramic material, such as an x-ray scintillator material.
Medical diagnostics and industrial inspection require high spatial resolution detection of x-rays transmitted through a body. For example, in a typical computed tomography (CT) scanning system, an x-ray source and an x-ray detector array are positioned on opposite sides of the subject and rotated around the subject in fixed relation to each other. In a CT scanning system using a solid scintillator, the scintillator material of a cell or element absorbs x-rays incident on that cell and emits light which is collected by a photodetector for that cell. Thus, the x-rays, the electromagnetic radiation of interest, stimulate or excite the scintillator material, which then emits scintillating radiation, e.g., light. During data collection, each cell or element of the detector array provides an electrical output signal representative of the present light intensity in that cell of the array. These output signals are processed to create an image of the subject in a manner which is well known in the CT scanner art.
It is desirable to absorb substantially all of the incident x-rays in the scintillator material in order to minimize the x-ray dose to which the body must be exposed during the diagnostic or inspection x-ray measurement. In order to collect substantially all of the incident x-rays, the scintillator material must have a thickness in the direction of x-ray travel which is sufficient to stop substantially all of the x-rays. This thickness depends both on the energy of the x-rays and on the x-ray stopping power of the scintillator material.
As the thickness of the scintillator increases, its transparency to the generated light must also increase so that substantially all of the light generated by the x-rays in the scintillator is collected by the photodetector. Collecting all the light generated by the x-rays maximizes overall system efficiency, the signal to noise ratio, and the accuracy with which the quantity of incident stimulating radiation, i.e., x-rays, may be measured. Furthermore, as the scintillator thickness increases, light generated near the scintillator surface opposite the photodiode has a relatively long distance to xe2x80x9cspread outxe2x80x9d. The xe2x80x9cspreading outxe2x80x9d of the light means that the light generated by an x-ray impinging upon the scintillator region directly on a particular photodetector cell may not be detected by that particular photodetector cell, but instead by an adjacent cell. Thus, the spatial resolution of the detector is reduced.
One method used to overcome these problems involves making the scintillator out of glass which can be drawn into fiber bundles. Fiber cladding and interspersed dark fibers can reduce the light spread and thereby improve spatial resolution. However, one disadvantage of this material is that due to the amorphous structure, glass scintillators have inherently low efficiencies of energy conversion of the stimulating x-rays to visible light. The efficiency of an x-ray scintillator material is the percentage of the energy of the absorbed x-rays which is generated as light. Glass scintillators also have relatively poor scintillation properties such as afterglow and radiation damage which limits their utility.
Afterglow in an x-ray detecting scintillator is the phenomena that luminescence from the scintillator due to x-ray excitation can still be observed a long time after the x-ray radiation is absorbed by the scintillator. Upon absorbing x-ray radiation the scintillator will emit light where the intensity of the light decays rapidly at an exponential rate. Additionally, the scintillator will emit a lower intensity light where the light intensity decays much more slowly. The more slowly decaying light is termed afterglow.
Radiation damage in an x-ray scintillator material is the characteristic of the scintillator material in which the quantity of light emitted by the scintillator material in response to the stimulating x-ray radiation changes after the material has been exposed to a high radiation dose.
In view of the foregoing, it would be desirable to provide a composite optically nonisotropic material which would increase the spatial resolution detection of a photodetector. According to one embodiment of this invention, an optically nonisotropic optical material is provided, which comprises two materials. The composite material includes a transparent bulk optical material, and fibers embedded within the transparent bulk optical material. The transparent bulk optical material may be a polycrystalline material, for example. The fibers are light absorbing or reflecting and substantially parallel to one another and thereby advantageously tend to channel light along the direction of the fibers.
In an embodiment of the invention where the composite optically nonisotropic material is a scintillator material, it would be desirable to provide a material with good energy conversion efficiency, afterglow, and radiation damage properties.
According to another embodiment of the invention, a high spatial resolution x-ray device is provided. The x-ray device includes an x-ray source and an optically nonisotropic composite scintillator material. The optically nonisotropic composite scintillator includes a transparent bulk scintillator material which absorbs x-ray radiation and emits scintillating radiation. The optically nonisotropic composite scintillator further includes fibers embedded with the bulk scintillator material where the fibers are substantially parallel along a direction. The x-ray device has a scintillating radiation detector optically coupled to the composite scintillator for detecting scintillating radiation from the bulk scintillator material. Advantageously, the composite scintillator will tend to channel scintillating light along the fibers and towards the scintillating radiation detector.
According to yet another embodiment of the invention, a high spatial resolution electromagnetic radiation detection device is provided. The radiation detection device includes an optically nonisotropic composite scintillator which comprises a transparent bulk scintillator and fibers embedded within the bulk scintillator. The fibers reflect or absorb scintillating radiation from the bulk scintillator and tend to channel the scintillating radiation towards a scintillating radiation detector optically coupled to the composite scintillator.
According to yet another embodiment of this invention, a process of making an optically nonisotropic composite scintillator material is provided. Powder is formed around sacrificial fibers. The powder and sacrificial fibers are then pressed to form a sacrificial fiber-powder compact. The sacrificial fibers are removed leaving holes in the compact. The powder compact is sintered to form a transparent polycrystalline ceramic, where the transparent polycrystalline ceramic is a scintillator. The holes of the transparent polycrystalline ceramic are filled with a fiber material, where the fiber material in the holes absorbs or reflects scintillating radiation from the transparent polycrystalline ceramic.